Cutting tools have been used in both coated and uncoated conditions for machining various metals and alloys. In order to increase cutting tool wear resistance and lifetime, one or more layers of refractory materials have been applied to cutting tool surfaces. TiC, TiCN, TiOCN, TiN and Al2O3, for example, have been applied to cemented carbide substrates by CVD.
Al2O3 or alumina has been of particular interest as a coating for cutting tools. Alumina demonstrates various crystalline phases including α, κ, γ, β AND θ with the α and κ phases occurring most frequently on CVD coated cemented carbide cutting tools. α-alumina has proven to be a desirable coating given its thermodynamic stability and general chemical inertness at various temperatures encountered in cutting applications. However, deposition of α-alumina is increasingly difficult and often sensitive to deposition conditions and the chemical identity of the cutting tool substrate. Prior literature, for example, has demonstrated the requirement to sufficiently oxidize titanium carbide surfaces prior to alumina deposition in order to induce formation of the α-phase. Insufficient oxidation of TiC surfaces results in the production of κ-alumina or a mixture of κ and α phases (see e.g., Vourinen, S., Thin Solid Films, 193/194 (1990) 536-546 and Layyous et al., Surface and Coatings Technology, 56 (1992) 89-95).
Moreover, α-alumina coatings on cemented carbide substrates have displayed significant adhesion problems leading to premature coating failure by delamination and other degradative pathways. Adhesion problems in α-alumina coatings on cemented carbides has been attributed to significant interfacial porosity developed between the coating and substrate during α-alumina deposition (see e.g., Chatfield et al., Journal de Physique, Colioque C5, supplement au n° 5, Tome 50, mai 1989).
In view of these findings, α-alumina bonding layers have been extensively researched and developed. The bonding layers are provided on surfaces of cemented carbide substrates to ensure the nucleation and growth of α-alumina and to mitigate or eliminate the development of interfacial porosity (see e.g., Halvarsson et al., Surface and Coatings Technology, 68/69 (1994) 266-273 and Halvarsson et al., Surface and Coatings 
Technology 76/77 (1995) 287-296).
Cutting tools based on polycrystalline cubic boron nitride (PcBN) are becoming more popular given the high hardness and high thermal stability (up to about 980° C.) of cBN. PcBN cutting tools, for example, find application in machining case-hardened and through-hardened steels, superalloys, chilled cast iron and gray cast iron. Additionally, PcBN based cutting tools are operable to run dry for clean machining processes thereby saving coolant, maintenance and disposal costs.
Similar to cemented carbides, cutting tools based on PcBN substrates can also benefit from the application of various refractory coatings. PcBN cutting tool substrates, for example, have been provided with TiCN, TiOCN, TiN and Al2O3 coatings. Nevertheless, as with cemented carbides, the deposition of α-alumina on PcBN cutting tool substrates occurs through the use of one or more intervening layers over the substrate, including bonding or modification layers. U.S. Pat. No. 7,455,918 to Gates et al. discloses PcBN cutting tools wherein α-alumina is deposited on modification layers residing between the PcBN substrate and the α-alumina layer.